From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-9.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 73EDFC4727F for ; Fri, 25 Sep 2020 10:11:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2CD9222211 for ; Fri, 25 Sep 2020 10:11:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727901AbgIYKLn convert rfc822-to-8bit (ORCPT ); Fri, 25 Sep 2020 06:11:43 -0400 Received: from smtp.h3c.com ([60.191.123.56]:10004 "EHLO h3cspam01-ex.h3c.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727132AbgIYKLm (ORCPT ); Fri, 25 Sep 2020 06:11:42 -0400 Received: from DAG2EX08-IDC.srv.huawei-3com.com ([10.8.0.71]) by h3cspam01-ex.h3c.com with ESMTPS id 08PABJ8p029846 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=FAIL); Fri, 25 Sep 2020 18:11:20 +0800 (GMT-8) (envelope-from tian.xianting@h3c.com) Received: from DAG2EX03-BASE.srv.huawei-3com.com (10.8.0.66) by DAG2EX08-IDC.srv.huawei-3com.com (10.8.0.71) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.1713.5; Fri, 25 Sep 2020 18:11:23 +0800 Received: from DAG2EX03-BASE.srv.huawei-3com.com ([fe80::5d18:e01c:bbbd:c074]) by DAG2EX03-BASE.srv.huawei-3com.com ([fe80::5d18:e01c:bbbd:c074%7]) with mapi id 15.01.1713.004; Fri, 25 Sep 2020 18:11:23 +0800 From: Tianxianting To: "axboe@kernel.dk" CC: "linux-block@vger.kernel.org" , "linux-kernel@vger.kernel.org" Subject: RE: [PATCH] [v2] blk-mq: add cond_resched() in __blk_mq_alloc_rq_maps() Thread-Topic: [PATCH] [v2] blk-mq: add cond_resched() in __blk_mq_alloc_rq_maps() Thread-Index: AQHWjMtkn2n+ezztLkuMBSittKHg9Kl5LeiA Date: Fri, 25 Sep 2020 10:11:22 +0000 Message-ID: <7ea73e3f24c9429993357a6e48db5a6b@h3c.com> References: <20200917081311.11428-1-tian.xianting@h3c.com> In-Reply-To: <20200917081311.11428-1-tian.xianting@h3c.com> Accept-Language: en-US Content-Language: zh-CN X-MS-Has-Attach: X-MS-TNEF-Correlator: x-originating-ip: [10.99.141.128] x-sender-location: DAG2 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 8BIT MIME-Version: 1.0 X-DNSRBL: X-MAIL: h3cspam01-ex.h3c.com 08PABJ8p029846 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Jens Could I get your comments for the issue? Whether it is a problem? Maybe my understanding is superficial, highly appreciated if you can comment a bit. -----Original Message----- From: tianxianting (RD) Sent: Thursday, September 17, 2020 4:13 PM To: axboe@kernel.dk Cc: linux-block@vger.kernel.org; linux-kernel@vger.kernel.org; tianxianting (RD) Subject: [PATCH] [v2] blk-mq: add cond_resched() in __blk_mq_alloc_rq_maps() We found it takes more time of blk_mq_alloc_rq_maps() in kernel space when testing nvme hot-plugging. The test and anlysis as below. Debug code, 1, blk_mq_alloc_rq_maps(): u64 start, end; depth = set->queue_depth; start = ktime_get_ns(); pr_err("[%d:%s switch:%ld,%ld] queue depth %d, nr_hw_queues %d\n", current->pid, current->comm, current->nvcsw, current->nivcsw, set->queue_depth, set->nr_hw_queues); do { err = __blk_mq_alloc_rq_maps(set); if (!err) break; set->queue_depth >>= 1; if (set->queue_depth < set->reserved_tags + BLK_MQ_TAG_MIN) { err = -ENOMEM; break; } } while (set->queue_depth); end = ktime_get_ns(); pr_err("[%d:%s switch:%ld,%ld] all hw queues init cost time %lld ns\n", current->pid, current->comm, current->nvcsw, current->nivcsw, end - start); 2, __blk_mq_alloc_rq_maps(): u64 start, end; for (i = 0; i < set->nr_hw_queues; i++) { start = ktime_get_ns(); if (!__blk_mq_alloc_rq_map(set, i)) goto out_unwind; end = ktime_get_ns(); pr_err("hw queue %d init cost time %lld\n", i, end - start); } Test nvme hot-plugging with above debug code, we found it totally cost more than 3ms in kernel space without being scheduled out when alloc rqs for all 16 hw queues with depth 1024, each hw queue cost about 140-250us. The time cost will be increased with hw queue number and queue depth increasing. And if __blk_mq_alloc_rq_maps() returns -ENOMEM, it will try "queue_depth >>= 1", more time will be consumed. [ 428.428771] nvme nvme0: pci function 10000:01:00.0 [ 428.428798] nvme 10000:01:00.0: enabling device (0000 -> 0002) [ 428.428806] pcieport 10000:00:00.0: can't derive routing for PCI INT A [ 428.428809] nvme 10000:01:00.0: PCI INT A: no GSI [ 432.593374] [4688:kworker/u33:8 switch:663,2] queue depth 30, nr_hw_queues 1 [ 432.593404] hw queue 0 init cost time 22883 ns [ 432.593408] [4688:kworker/u33:8 switch:663,2] all hw queues init cost time 35960 ns [ 432.595953] nvme nvme0: 16/0/0 default/read/poll queues [ 432.595958] [4688:kworker/u33:8 switch:700,2] queue depth 1023, nr_hw_queues 16 [ 432.596203] hw queue 0 init cost time 242630 ns [ 432.596441] hw queue 1 init cost time 235913 ns [ 432.596659] hw queue 2 init cost time 216461 ns [ 432.596877] hw queue 3 init cost time 215851 ns [ 432.597107] hw queue 4 init cost time 228406 ns [ 432.597336] hw queue 5 init cost time 227298 ns [ 432.597564] hw queue 6 init cost time 224633 ns [ 432.597785] hw queue 7 init cost time 219954 ns [ 432.597937] hw queue 8 init cost time 150930 ns [ 432.598082] hw queue 9 init cost time 143496 ns [ 432.598231] hw queue 10 init cost time 147261 ns [ 432.598397] hw queue 11 init cost time 164522 ns [ 432.598542] hw queue 12 init cost time 143401 ns [ 432.598692] hw queue 13 init cost time 148934 ns [ 432.598841] hw queue 14 init cost time 147194 ns [ 432.598991] hw queue 15 init cost time 148942 ns [ 432.598993] [4688:kworker/u33:8 switch:700,2] all hw queues init cost time 3035099 ns [ 432.602611] nvme0n1: p1 So use this patch to trigger schedule between each hw queue init, to avoid other threads getting stuck. We call cond_resched() only when "queue depth >= 512". We are not in atomic context when executing __blk_mq_alloc_rq_maps(), so it is safe to call cond_resched(). Signed-off-by: Xianting Tian --- block/blk-mq.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/block/blk-mq.c b/block/blk-mq.c index b3d2785ee..5a71fe53a 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -3255,11 +3255,16 @@ void blk_mq_exit_queue(struct request_queue *q) static int __blk_mq_alloc_rq_maps(struct blk_mq_tag_set *set) { int i; + unsigned int depth = set->queue_depth; - for (i = 0; i < set->nr_hw_queues; i++) + for (i = 0; i < set->nr_hw_queues; i++) { if (!__blk_mq_alloc_map_and_request(set, i)) goto out_unwind; + if (depth >= 512) + cond_resched(); + } + return 0; out_unwind: -- 2.17.1